The long-term objective of this research plan is to develop a stereotactic operating microscope system by which the capability, precision, and safety of neurosurgical operative technique is advanced. This system will store, spatially register, reformat, and project into the optics of the operating microscope information from multiple imaging modalities (CT, angiography, MRI) and from an anatomic atlas such that the surgeon will see superimposed upon the operative field that information in proper position, orientation, and scale. Unique to this system is the adaptation of a non-imaging ultrasonic three-dimensional digitizing localizer to register the operating microscope, the patient, and the imaging/atlas information in a common coordinate space. The operating microscope remains freely moveable and without mechanical linkage to the operative field; the operative field remains unencumbered by conventional stereotactic, ultrasound, or other guidance equipment. This system will achieve an accuracy on the order of one millimeter by refinement of the acoustic localizing component and its registration algorithms. Concurrently, the data bank (containing CT information in the prototype successfully developed to demonstrate the feasibility of the concept) will be expanded to include angiographic, MRI, and anatomic atlas information by development of appropriate spatial registration software. Processing and display of integrated preoperative data will be facilitated by upgrading of the operating room computer and optical projection component; development of interactive computer graphics programs will allow preoperative treatment planning. The utility and potential value of this operative guidance system will be assessed clinically in the operating room during intracranial microsurgical procedures (e.g. partial and total tumor resection); potential extracranial applications will be explored.